Università Ca’ Foscari Venezia
Dottorato di Ricerca in Scienze Ambientali, Ciclo XXII (A. A. 2006/2007 – A.A. 2008/2009)
COASTAL PLAIN IN NORTHEASTERN BUENOS AIRES PROVINCE: HYDROGEOLOGICAL CHARACTERISTICS
SETTORE SCIENTIFICO DISCIPLINARE DI AFFERENZA: GEO/05 Tesi di dottorato di Jerónimo Enrique Ainchil, matricola 955399
Coordinatore del dottorato Tutore del dottorando
Prof. Bruno Pavoni Prof. Giovanni Maria Zuppi
Co-tutore del dottorando Dr. Andrea Mazzoldi
RIASSUNTO
I concetti di sviluppo sostenibile, gestione integrata delle aree costiere, iniziative come la “Ramsar Convention”, e più in generale tutti gli sforzi indirizzati alla conservazione ed al recupero ambientale richiedono molteplici informazioni per un’attività decisionale. In questo contesto è essenziale comprendere il comportamento delle componenti ambientali e la loro interazione.
L’acquisizione di dati e la elaborazione delle informazioni sono generalmente processi costosi. Inoltre, possono richiedere lunghi tempi di esecuzione. La tesi qui di seguito esposta prevede una sequenza di passi successivi, in cui ogni passo porta ad una maggiore comprensione dei differenti aspetti della idrologia nella zona costiera in NE di Buenos Aires (Argentina), convergendo in una proposta di modello concettuale idrogeologico. Questo modello è la base necessaria per l’assunzione delle decisioni relative allo sviluppo economico e sociale dell’area in esame.
Partendo da una indagine geofisica, è necessaria la realizzazione di perforazioni per verificare la litologia del sistema acquifero. In seguito, viene costruita una rete di monitoraggio per mezzo della quale i dati di livello portano alla conoscenza delle condizioni di flusso e delle relazioni nella dinamica tra i livelli degli acquiferi. Questi pozzi per misure freatiche permettono di ottenere campioni d’acqua che, analizzati, caratterizzano le acque del sistema. Inoltre, è possibile ricostruire la storia delle acque attraverso gli isotopi ambientali.
Le conclusioni evidenziano il contributo di ogni tecnica adottata alla costruzione di un modello ed i vantaggi derivanti dall’integrazione delle informazioni ottenute.
Alla fine, si può concludere che è fondamentale conoscere le caratteristiche delle risorse naturali per indirizzare le politiche di sviluppo regionale.
ABSTRACT
The concepts of sustainable development, integrated management of coastal zones, initiatives such as the Ramsar Convention, and in general all the efforts aimed at environmental conservation and recovery require information for decision-making. In this context, it is essential to understand the behaviour of environmental components and their interrelation.
Data acquisition and information processing are usually expensive processes. Besides, they can last long periods of time. This thesis involves a sequence in which each step makes it possible to advance in better understanding of the different aspects of the coastal zone in NE of Buenos Aires (Argentina) hydrogeology, concluding in the proposal of a model. This model is the necessary basis for decision-making related to economic and social development of the analysed area. This area has an important industry activity.
Starting from a geophysical survey, the construction of boreholes is undertaken in order to verify the lithological sequence of the aquifer system. Afterwards, a monitoring network is constructed, and data of levels enabling knowledge of flow conditions and the dynamic relation of aquifer levels are collected. These boreholes also enable us to obtain samples to be analysed and that characterise waters in the system. Moreover, it is possible to reconstruct the history of waters with environmental isotopes.
The conclusions highlight the contribution of each technique to the construction of a model and the advantages of constructing a model integrating the information processed by the methodologies used.
Finally, it should be noted that it is significant to know the features of natural resources in order to establish regional development policies.
ACKNOWLEDGMENTS
The author wishes to thank the following people and institutions:
Professors Giovanni Maria Zuppi and Bruno Pavoni, Universitá Ca’ Foscari di Venezia
Dr Andrea Mazzoldi, ISMAR-CNR - Venezia
Dr Luigi Tosi, Dra Federica Rizzetto and Dr Maurizio Bonardi, ISMAR-CNR - Venezia
Professor Eduardo Kruse, Universidad Nacional de La Plata, Argentina Dr Sebastián Pera, DACD-IST-SUPSI - Switzerland
Dr Daniel Nieto Yabar, OGS - Trieste Marco Giada, Morgan S.r.L. – Venezia
Dra Cristina Dapeña, INGEIS, Universidad de Buenos Aires, Argentina Nicoletta, Eloissa and Samanta
All the CNR – ISMAR – Venezia - SS Apostoli´s staff ALFA – Europe Aid – Co operation office
INDEX
1. INTRODUCTION AND SCOPE
2. GENERAL CHARACTERISTICS OF THE AREA 2.1 Location
2.2 Social and economic aspects 2.3 Climate
2.4 Soil
2.5 Surface water resources
3. GEOLOGICAL SETTING
3.1 Regional and structural geology 3.2 Geomorphology
3.3 Stratigraphy 3.4 Hydrogeology
4. MATHERIALS AND METHODS. RESULTS 4.1 Geophysical survey
4.1.1. Vertical Electrical Sounding 4.1.2 Electrical Imaging
4.2 Geological profile 4.3 Monitoring network 4.4 Level variation 4.5 Groundwater flux
4.6 Groundwater chemical features
4.6.1 Chemical characteristics of the phreatic layer 4.6.2 Chemical characteristics of Pampeano and Puelche aquifers
4.6.3 Minor ions and trace elements 4.7 Environmental isotope results
4.7.1 Stable isotopes 4.7.2 Tritium 1 6 6 7 10 16 17 19 19 21 27 28 33 34 35 42 49 55 63 66 70 71 82 89 95 97 103
5. DISCUSSION 6. CONCLUSIONS 7. THEORETICAL BACKGROUND 8. BIBLIOGRAPHY 9. ANNEXES 107 111 116 182 195
1. INTRODUCTION AND SCOPE
It is known that water is one of the bases of life as well as a resource of primary importance for the socioeconomic development of a region and the preservation of its environmental characteristics. Understanding how water behaves and conducting a relevant follow-up is becoming more important every day, not only to correctly plan the use of water but also to anticipate any quali-quantitative alteration of this resource.
Problems derived from the pollution of water resources have led to acknowledge the need to advance in the scientific knowledge of global hydrological aspects with an interactive view from all the disciplines involved in the study of this resource (Eagleason 1991).
Regions with less socioeconomic development, which are more frequent in new countries like Argentina, are usually characterised by deficiency in the understanding of groundwater behaviour. This is especially due to a lack of information about the physical media related to the subsoil (deep drilling) and about measurements of the hydrodynamic and hydrochemical system. It is assumed that, because of its recent history and natural conditions, a new country is one in which man is being faced with an incipient "crisis" with natural resources, becoming aware of the importance of knowing more about them (Room et al 1983).
The previous concepts applied to our country cannot be considered without the hydrological issue of big plains. Generally, the less known characteristics in flat environments are:
a) predominance of water vertical movements (evapotranspiration -infiltration) over horizontal ones (runoffs)
b) a strong relationship between surface water and groundwater in every hydrological process.
The most recent advances focused on this (Kruse and Zimermann, 2002) because groundwater is an important element in environmental
matters due to the quantification of processes of infiltration, evapotranspiration, water transport in the non-saturated zone (NSZ) and in the saturated zone (SZ).
Scientific research should at first contemplate a global analysis of a region and adapt to different spatial and temporary scales. Such research and an integrated treatment of the cycle (surface water, groundwater, hydrometeorological variables) are essential to take actions towards a balance between maintained socioeconomic development, water needs and the environmental protection of plains. The topics covered in this thesis tend to satisfy precisely these requirements according to three working focal points:
Regional characterisation of hydrological processes
Quantification and modelling of the above-mentioned processes at basin level
Detailed description of methodologies or technologies for measuring the characteristics of these processes
Different methodologies and techniques, some regarded as traditional and some as innovative, have been used to study the region. Geophysical exploration, hydrogeochemical characterisation, environmental isotope identification and drilling construction have been included. Synergic results, contributing to an integrated model for the area of study, have been obtained from the combination of these techniques.
The area of study comprises the coastal plain of Río de La Plata (Argentina). The urban process began 100 years ago in this area. Urbanisation modified natural conditions: elevation of natural terrain and changes in natural drainage, industries and city establishment. Such coastal plain, with still almost the same natural conditions, was investigated in the north of this area by Pera (2004).
In the region under study the current conditions include intensive land use, groundwater overexploitation, solid waste disposal and industrial issues. These conditions have produced different effects, which will be analysed later on:
Reduction of pluviometric infiltration
Reduction of evaporation and transpiration phenomena Increase in surface water streams
Groundwater chemistry quality alteration
Finally, it should be noted that Integrated Coastal Zone Management (ICZM) has been designed to ‘join up’ all the policies that have an effect on coastal regions. It deals with both planning and management of coastal resources and coastal space. ICZM is not just an environmental policy. Although the need to protect natural ecosystem functioning is one of its key aims, ICZM also seeks to improve the economic and social well-being of coastal zones and help them develop their full potential as modern, vibrant communities. In the coastal zone, these environmental and socioeconomic goals are intrinsically interconnected.
All the management activities related to subsurface resources require the understanding of its features, characteristics and properties.
Objetives
The overall objective of this thesis is to describe groundwater behaviour in the area of study. This behaviour is essential for defining environmental management criteria for regional sustainable development.
The specific objectives are:
To use geophysical methods to determine groundwater characteristics, especially to recognise soil composition.
To perform boreholes to verify hydrogeological conditions. To install and operate a groundwater monitoring network. To describe the main features of groundwater level variations. To analyse hydrochemical features of aquifer units.
To isotopically characterise groundwater.
To deepen our knowledge of the behaviour of infiltration, flux, and contamination in natural conditions or in areas with human activities.
To use several techniques to obtain an accurate model. To formulate a conceptual model of groundwater behaviour.
Organisation of the thesis
This thesis is divided into seven chapters that approach each thematic unit.
Chapter 1 is an introduction.
Chapter 2 introduces the area of study. First, the geographical location is included. Second, social and economic characteristics are discussed. Third, the physical features of the region are described: weather, soils and surface water resources. This chapter has been based on a compilation of background information.
Chapter 3 deals with regional geological and hydrogeological characteristics in terms of an analysis of the compiled precedents.
Chapter 4 describes all the tasks undertaken here for each of the topics mentioned above. Two types of objectives are distinguished in the geophysical survey: one exploring the lithological section of interest in all its depth and the other dealing with high-definition shallow
subsurface. The techniques used for analytical determination, limit of quantification, sampling standards and display of graphical results are included with chemical data. Origin and estimation of groundwater age is recognised with the application of isotopes.
Chapter 5 discusses the results as a whole and introduces a model. Chapter 6 displays the conclusions and recommendations.
Chapter 7 concerns the theoretical background and methodologies used throughout this thesis. First, the geophysical methods employed here are included. Second, the hydrogeochemical concepts are described. And finally, the application of environmental isotopes to study groundwater evolution is summarised.
2. GENERAL CHARACTERISTICS OF THE AREA
2.1 Location
The area of study is located in the República Argentina in South America (Figure 2.1.1)
Figure 2.1.1 República Argentina in South America
in the northeastern region of the Province of Buenos Aires (Figure 2.1.2)
in the cities of Berisso and Ensenada, close to La Plata city (Figure 2.1.3)
Figure 2.1.3 Cities of Berisso and Ensenada
2.2 Social and economic aspects
The moderate weather and the particular location of the region are ideal for business settling because of its proximity to the largest Argentine population and industrial conglomerate, called Conurbano Bonaerense (the suburbs of Buenos Aires) and to Ciudad Autónoma de Buenos Aires (Buenos Aires city). This city has excellent road and rail links as well as seaways and airways. La Plata - Buenos Aires motorway and Autovía 2 (dual carriageway ) is very important roadworks. Ferrocarril General Roca (railway line), Puerto La Plata (port) and an airport are also important links.
The area of study has an intermediate population density (66.1 people per km2), lower than the average of Conurbano Bonaerense but higher than the provincial average (46.7 people per km2). The spatial configuration of the region is well-balanced. La Plata (620.3 people per km2) like any capital city of a province, is located among intermediate cities with neighbouring rural areas and has plenty of artistic, cultural and scientific events to offer.
0 1000 2000 m
A high proportion of population at a potentially active age is concentrated in La Plata, Berisso and Ensenada districts. This is probably due to migratory phenomena, the attraction exerted by universities or demand for both industry and public sector jobs.
Access to the area
Road access. The primary routes that run through the region are the national routes 2, 3 and 11 and the provincial ones, 6 and 205. It is also important to mention La Plata - Buenos Aires motorway, which connects both cities as well as Berazategui, Avellaneda and Quilmes districts. In addition to connecting those two cities, route 2 links the region under study with Mar del Plata city going through important localities.
Fluvial and maritime access. The area of study includes Puerto La Plata. Inaugurated in 1890, this port was built pursuant to the political decision to found a city with the same name as the new capital of the Province of Buenos Aires in 1882. Although the port is provincial, the area of main influence coincides with the region under study. Over many decades, the port has been the epicentre of intense economic activity in the region and boosted the development of the cities of La Plata, Ensenada and Berisso. The port has had numerous uses: liquid and general cargos, with a great deal of movement of meat exports. The advantage of Puerto La Plata over other ports in the region is its connection and ferry services.
Free Trade Zone. La Plata Free Trade Zone is located in a piece of land bounded by Puerto La Plata and Astillero Río Santiago, covers a surface area of 70 hectares and has good road and maritime access. This strategic location has turned the Free Trade Zone into an international distribution and logistics centre with about 180 currently settled direct users.
Railway access. Ferrosur Roca S.A. is the cargo operator that connects the main production centres in the south and east of Buenos Aires Province. Most non-metallic minerals and building materials are transported by the national railway system through Ferrosur Roca from the cement plants in Olavarría. In fact, in 2005, 65% of the total freight of such branch line comprised this type of product.
Educational level
Overall, the region shows very good educational indicators compared to the rest of the Province of Buenos Aires and the country. The percentage of students attending educational institutions is higher than the average of the province and country, especially in basic education, that is, education for three- to fourteen-year-olds.
A high attendance percentage is also observed in the group encompassing fifteen- to seventeen-year-olds. This percentage equals or surpasses the average of the whole country.
The age group receiving higher education reveals very good indicators, too. La Plata should be highlighted since it easily surpasses the percentages of the country and the whole Province of Buenos Aires.
Regional industry. General Characteristics
Analysing the gross value of industrial output (GVIO) per district in 1993, a high concentration is observed in Ensenada district (66%),
followed by La Plata districts. The sector of chemical products, by-products of oil, carbon, rubber and plastics accounts for 67.3% of GVIO of the region. Most of the activity involved in this sector takes place in Ensenada district, the main petrochemical complexes in Argentina. Moreover, this activity is the most important one in La Plata district.
The basic metal manufacturing industry represents 11% of the region and is especially concentrated in Ensenada district. On the other hand, food, beverage and tobacco production represents 10.9% of the region and La Plata district is the most representative of this activity.
To sum up, the processes of urbanisation and industrialisation in the analysed region have taken place over a period of more than 100 years.
2.3 Climatic characterisation
The region is subject to mild and wet climate, with average annual rainfall that is slightly higher than 1000 mm and an average annual temperature of 15.7º C. The data obtained within the period 1991-2000 from the reference meteorological station that is closest to the project's area (No. 87593 of the World Meteorological Organization, WMO) was used to describe the area in terms of climate. These data belong to La Plata Aero (National Weather Service) and have the following geographical coordinates: Latitude: - 34.58º S. Longitude: - 57.54º W. Altitude: 19 metres a.m.s.l.
Precipitation
Figure 2.3.1 shows the average monthly precipitation from 1991 to 2000.
Figure 2.3.1. Average monthly precipitation
Although rainfall is irregularly distributed throughout the year, its intensity is higher between October and April. The total average reaches 1079.3 mm/yr. Summer and autumn are the most humid seasons and winter is the driest one in the rainfall pattern. Mean seasonal precipitation (1991-2000) is distributed according to the values shown in Figure 2.3.1, with its maximum in April (113.1 mm) and its minimum in August (57.5 mm).
Temperature
The average annual temperature from 1991 to 2000 is 15.7º C. Figure 2.3.2 shows the maximum, minimum and mean temperatures of La Plata. In summer, the highest mean temperature value is registered in January: 21.2º C. The maximum mean monthly value is 24.4º C and is registered in January whereas the minimum mean monthly temperature is 6.3º C and is registered in July.
AVERAGE MONTHLY PRECIPITATION (1991-2000) 0.0 20.0 40.0 60.0 80.0 100.0 120.0 1 2 3 4 5 6 7 8 9 10 11 12 Month mm
Figure 2.3.2 Maximum, minimum and mean monthly temperatures (La Plata Aero Reference Station).
Winds
Figures 2.3.3, 2.3.4 and 2.3.5 show the mean monthly wind speed and the mean wind speed and frequency in terms of direction.
Figure 2.3.3. Mean wind velocity (La Plata Aero Reference Station)
The mean annual wind speed is 18.9 km/h. The maximum velocity has been recorded in August, with 25.9 km/h, followed by that of February,
MEAN WIND SPEED (1991-2000) 0.0 5.0 10.0 15.0 20.0 25.0 1 2 3 4 5 6 7 8 9 10 11 12 Month km/ h
MONTHLY MEAN TEMPERATURE (years 1991-2000) 0.0 5.0 10.0 15.0 20.0 25.0 30.0 1 2 3 4 5 6 7 8 9 10 11 12 Month ºC T TM Tm
with 24.1 km/h. The highest frequency of winds comes from the east and northeast directions.
Figure 2.3.4. Mean wind velocity in terms of direction (La Plata Aero Reference Station)
Figure 2.3.5. Wind direction frequency in terms of direction (La Plata Aero Reference Station)
MEAN WIND VELOCITY
0 5 10 15 20 25 N NE E SE SE SW W NW
Mean wind velocity
WIND DIRECTION FREQUENCY
0 50 100 150 N NE E SE SE SW W NW Direction Frequency
Atmospheric pressure
The atmospheric pressure data shown in Table 2.3.1 belong to La Plata Aero Reference Station. In other words, these data are the pressure values at the station level in hectopascals (hPa). Mean atmospheric pressure ranges from 1008.5 hPa in December to 1017 hPa in July.
Table 2.3.1: Atmospheric pressure
AP JAN FEB MAR APR MAY JUN
(hPa) 1009.1 1011.0 1011.9 1012.8 1014.9 1014.2
AP JUL AUG SEP OCT NOV DEC
(hPa) 1017.0 1016.6 1015.7 1013.7 1011.2 1008.5
Relative humidity
Figure 2.3.6 shows the mean monthly relative humidity variations. The figures obtained here indicate an average of 76.73% throughout the year, with seasonal variations that do not differ from the aforementioned value in more than 10%.
Figure 2.3.6. Mean monthly relative humidity from La Plata Aero Station.
MEAN MONTHLY RELATIVE HUMIDITY (1991-2000) 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 1 2 3 4 5 6 7 8 9 10 11 12 Month %
Water budget
In order to estimate real evapotranspiration, Thornthwaite method was used (Thornthwaite y Mather, 1955). The values obtained are shown in Table 2.3.2.
Table 2.3.2. Water balance following Thornthwaite method (EVT: evapotranspiration)
SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG Total
temperature 12.5 15.5 18 21.2 22.1 21.2 20.2 16.1 12.9 9.8 8.3 10.6 Heat Index 4.00 5.55 6.95 8.91 9.49 8.91 8.28 5.87 4.20 2.77 2.15 3.12 70.21 Potential EVT 40.4 57.0 72.4 94.2 100.7 94.2 87.2 60.6 42.5 27.3 20.9 31.0 No. days month 30 31 30 31 31 28.3 31 30 31 30 31 31 No. daylight hours 12.5 11.2 10 9.4 9.7 10.6 12 13.3 14.4 15 14.7 13.7 Actual EVT 42.0 55.0 60.4 76.2 84.1 78.3 90.1 67.1 52.6 34.1 26.5 36.6 703.1 Precipitation 62.1 105.1 104.3 107.1 112.2 103.0 88.4 113.1 90.9 67.4 68.2 57.5 1079.3 EVT 42.0 55.0 60.4 76.2 84.1 78.3 90.1 67.1 52.6 34.1 26.5 36.6 703.1 Deficit 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Storage 0 20.1 50.0 50.0 50.0 50.0 50.0 48.3 50.0 50.0 50.0 50.0 50.0 Surplus 0.0 20.2 43.9 30.9 28.1 24.7 0.0 44.3 38.3 33.3 41.7 20.9 326.2
Figure 2.3.7 shows the monthly evolution of soil moisture storage and water surplus.
It is possible to carry out a climate classification that reflects a global characterisation of the relevant period with the results yielded from the mean monthly water budgets. According to these indexes, the climate of the region can be classified as B2C’2“r”a’, where B2 stands for humid climate and C’2 for microthermal climate (Thornthwaite, op. cit.). This indicates that the regional climate has PE values higher than 570 mm, with nil or little water deficiency (“r”) and a summer concentration percentage of thermal efficiency that is lower than 33.9% (a’). The region can be affected by variations within certain limits that will depend on the particular surplus and/or deficit occurrence of each year.
Normally the climatic characteristics indicate that there is water excess that can cause infiltration, runoff and depress storage.
2.4 Soil. General conditions
Three overlapping materials of different origin can be distinguished in the natural ground of the region:
a) In the surface, approximately from 0.80 to 1.50 m deep, there is highly clayey material, possibly of mixed origin (fluvial and marine), with distinctive contraction-expansion features revealed by abundant landslides and crevices. Traces of clay illuviation (coatings) can be identified, despite being slighty hidden due to contraction - expansion features (Cappannini y Mauriño, 1966).
b) Below is marine origin material of about 1.00 m thick and sandy to sandy loam texture. A series of thick and thin layers mixed with pieces of small shells in different degrees of fragmentation are sometimes observed .
c) Beneath this material, there is intensely-crushed, dun-coloured massive loessial material, with calcium carbonate accumulation that looks like thick concretion, of loamy to silt loamy texture (Pampeano).
All grounds usually have drainage deficiencies due to surface flooding, frequently accompanied by phreatic level close to the surface. That is revealed by hydromorphic features (mottled, iron-manganese concretions) (Giménez et al. 1992). In most cases, the first two materials have high interchangeable sodium content and they sometimes have soluble salts, too.
Because of its drainage deficiency, the highly clayey texture of the soil and its high content of interchangeable sodium and sometimes soluble salts, the ground is suitable only for cattle raising and afforestation with adapted species.
2.5 Surface water resources
In the coastal plain, where the project is being developed, river beds stray, disappearing into Bañado Maldonado and Bañado de Ensenada (marsh). Watersheds practically disappear because of the flat relief and in most cases discharge into Río de La Plata can only be achieved through canalisations.
Most streams are perennial or permanent in the lower stretches of their basins due to underground contribution (Varela et al, 2002). However, in their medium and high stretches, streams become intermittent since their basins are located above phreatic surface. Data from existing measurements (Auge, 1995) show a runoff rate of 6% compared to the rain, with mean water level ranging from 30 to 70 L/s. In the coastal plain, water is most frequently driven into Río de La Plata through canalisations, which have a regional discharge direction towards the northeast.
Due to the topography and drainage net mentioned above, these bodies do not have significant water levels. They constitute bodies of slow flow that can considerably augment in periods of pouring and increasing rain, which are affected by variations in tide and “sudestada”. When river levels rose sharply, most of the natural area is affected by flooding.
The area belonging to Ensenada marshlands naturally represents an area of groundwater discharge. The natural expansion and retraction of flooded surfaces, apart from rising surface water and “sudestadas”, is related to phreatic level rise and depression. Nevertheless, the temporary influence of heavy rain should be highlighted since marshlands additionally receive direct precipitation in the area and influx of courses located in the highest places.
In conclusion, phreatic variations are mainly related to weather conditions. There are both short fluctuations caused by rain and longer fluctuations caused by the alternation of dry and humid periods.
3. GEOLOGICAL SETTING
3.1 Regional and structural geology
The area is located in the Chaco-Pampeano basin on the passive margin of South America (Ramos, 1999), characterised by low tectonic and seismic activity since Mesozoic era (Russo et al, 1979). The basement outcropping on the northern coast readily submerges in a southward direction under the plains, located at -486 meters in La Plata, reaching the deepest point in Salado basin.
Salado basin is a depression that is perpendicular to the coast, genetically linked to the rift valley opened between Africa and South America giving rise to the Atlantic Ocean and being under subsidence for most of its history (Figure 3.1.1) (Rolleri, 1975). Over metamorphic basement and late-Jurassic lower-Cretaceous volcanic rocks, its sedimentary register goes from Middle-Cretaceous to Quaternary, with up to 6000 meters thick, disposed in an overlapping sequence. Deposits are continental and marine changing to exclusively marine origin in the continental shelf (Bracaccini, 1980; Tavella and Wright, 1996).
The upper sequence of the basin is composed of younger than Miocene sediments, which can be divided into three major units (Yrigoyen, 1975). The lowermost is composed of marine origin green clays, the intermediate is composed of fluvial origin sands on the continent changing to marine equivalent facies on the shelf, and the upper terms are represented by loess-like sediments on the continent changing to marine in an arrangement representing al least four different transgressive events (Parker et al, 1999; Violante and Parker, 1999).
Quaternary sea level variations due to glacial periods and eustatic movements deeply affected the geomorphology of the whole area, whose evolution towards the actual state started 2.4 million years ago, when a fluvial environment was installed (Parker et al, 1994). That was followed by successive transgressions and regressions during the Plio-Pleistocene before the post-last glacial maximum transgression (Violante and Parker, 1999).
Guilderson et al (2000) reconstructed a eustatic relative sea-level curve based on AMS 14C dated shells by resampling sediment cores previously analysed by Fray and Ewing (1963) and Parker and Violante (1982). The curve so obtained shows a minimum sea-level ca 16,690 years BP at 157 meters below present sea level, although those data are not conclusive about the position of the last glacial maximum (Violante and Parker, 2003). After subtracting tectonic effects, Guilderson et al (2000) concluded that eustatic component of the sea-level rise during the transgression following the last glacial maximum was 105 meters.
After the last glacial maximum, sea level began to rise. No evidence about changes in rise rate was found before 8,600 years BP. However, certain erosive features exposing Plio-Pleistocene marine sequences and interrupting the deposition of post-LGM sediments were found in the outer step of Río de La Plata terrace. This suggests an interruption on
the sea level rise (Violante and Parker, 2003). According to Cavalotto et al (1995), Violante and Parker, (2000), and Cavalotto (2003), between 8,600 and 6,000 years BP there was change in rise rate, slowing until it reached the altitude of +6 meters above present sea level, followed by a fall at different rates towards its current position.
3.2 Geomorphology
The area analysed in this work is located within the geomorphologic region known as Undulate Plain (Frenguelli, 1950), in the north of Buenos Aires Province (Figure 3.2.1). The boundaries of the region are:
NE and E: Río Paraná alluvial plain and Río de la Plata estuary.
N: Arroyo del Medio and High Plain (Province of Santa Fe).
W and SW: Dune Plain.
The southern boundary consists of the watershed with Río Salado basin (Depressed Plain).
The general area is characterised by having a dominant gradient towards NE and extreme altitudes of 30 meters a.m.s.l. in the watershed coincident with the SW boundary and of 0 meters on the banks of Río de La Plata. Extreme topographic gradients range from 1.3 to 0.7 meters/km.
Three main morphological components can be distinguished in the flat region mentioned above, namely, Coastal Plain, Low Plain and High Plain (Cappanini y Domínguez, 1961). Another component encompassing these can also be recognised: Coastal Cliff or Step. The area analysed in this project is located in the flat area of the coastal plain and at the foot of the cliff or step.
Figure3.2.1: Geomorphologic regions. The area of study is located within the Undulate Plain
Coastal dynamics of Río de La Plata
The river regime is influenced by discharge of its two main tributaries: Paraná and Uruguay. Their annual discharge is 16,000 m3/s and 6,000 m3/s respectively (C.A.R.P., 1989) and has no significant effect on Río de La Plata level. This river opens out into a wide estuary of approximately 35,000 km², where its levels are regulated by tides and the typical weather conditions: “sudestadas” and cold south winds push its water respectively towards the Argentine coast or the Uruguayan coast.
Tides: A usual tide has very little amplitude (0.46/0.52 m), belonging to a microtidal range. Syzygial mean heights range from 0.67 to -0.08 meters and the quadrature ones range from 0.55 to 0.01 m, decreasing inward (Servicio de Hidrografía Naval, 1993). Tide amplitude also decreases towards the Uruguayan coast, which is accounted for by the
Coriolis effect. The currents generated by these tides play an important role in modelling the riverbed.
Waves: The registered mean wave height is 0.31 meters (Halcrow, 1969). The highest waves occur in the outer sector and the lowest waves occur in the inner one. The wind direction caused by the greatest swells decreasingly blows ESE and E in the external stations whereas in the internal ones it blows E and ESE (C.A.R.P. 1992).
Littoral currents: Littoral currents are induced by waves and generate a flow parallel to the coast, with a resultant on the southern bank of Río de La Plata towards the northeast and southeast of Punta Piedras. These currents cause sediment transport (littoral drift) in opposite direction to that caused by river inflows circulating slightly further away from the shore (Fig.3.2.2.). The effect of the littoral currents is evident due to the definite northeast orientation of the fluvial valleys crossing Buenos Aires coastal plain and of inflowing suspended sediment plumes (Cavalotto, 2005).
Figure 3.2.2.: Satellite image showing the littoral drift in the coastal zone of Río de La Plata
The resulting effect of the factors mentioned above can be shown in current images of the coastal area at low tide, where megaripples are visible on bottom sediments and creeks that are subparallel to the coast. Figure 3.2.2 shows the drift current transport direction.
The coastal plain in the southern bank of Río de La Plata evolved on a modelled substratum during the Holocene transgression. Its current configuration resulted from the process of progradation that took place together with the last relative sea level drop (Parker et al, 1999).
The evolution of the coastal plain was controlled by the interaction of hydrometeorological conditions (wind-wave interaction, currents and tides), muddy depocentre migration, relative sea level fluctuations, and pre-Holocene surface geometry. Sea level geometry and fluctuations determined the distribution, extension and development of sedimentary sequences whereas hydrometeorological conditions established the measurement of sediment inflow and transport (Violante and Parker, 1999).
The most important feature of pre-Holocene surface is the old Río de La Plata fluvial valley. When the last sea level rise reached the top edge of the valley, Punta Piedras (a point located on the southern edge of the coastal plain) acted as the focus of southeastern waves. This triggered the formation of two littoral currents with opposite directions: one northwestward (towards Río de La Plata) and the other southwestward (towards Samborombón bay). Such conditions still exist today.
Coastal evolution is summarised in three moments: estuarine, coastal plain and fluvioestuarine delta, representing the processes of paleovalley fill, coastal progradation and installation of a deltaic system respectively (Cavalotto, 2005).
The coastal plain under study can be divided into the following units (Cavallotto, 2005):
Mud flat. A mud flat appears as a poorly drained flat and concave surface with development of unintegrated wetlands or marshes constituting, in some areas, a true hygrotope covered with scrubs. It consists of a sequence of coastal paleolines whose orientation implies a prograding sequence towards the northeast. It is interpreted as a sequence of wetlands originated in an environment associated with common conditions of the freshwater-saltwater interface.
Coastal levee: “It is a gentle small hill extended along the outer edge of the coastal plain, from the northern end of the area of study to Punta Blanca, except for the area interrupted by an erosive coastal. It consists of a sequence of parallel beach ridges that have grown in the same direction as the littoral drift (towards the NW), and hence enclose a low floodable area behind.
Local characterisation
This research work is done in the geomorphologic unit known as Coastal Plain (Figure 3.2.4). This unit spreads in a strip of 5 to 8 km wide, arranged parallel to Río de La Plata coastline (Fidalgo y Martínez, 1983).
Figure 3.2.3 Landsat image of the area of study
It is a flat relief located between an altitude of 5 meters a.m.s.l. and sea level. It has monotonous features and bad drainage with mean topographic gradients of 0.5 m/km (Laurencena et al, 2002). Surface watersheds do not practically occur here. There are depressed areas, where wetlands and poorly defined watercourses originate. This zone sometimes shows an anarchic drainage design. It contains a sequence of shapes that arose during the Holocene transgressive-regressive cycle.
The Coastal Plain shows several subunits, the largest being called mud flat (Cavallotto, 2005). It is such a scarcely drained flat and concave surface that unintegrated wetlands or marshes develop there. Sedimentary sequences include: a) sandy and clayey silts, b) greenish brown and green to yellowish/greyish green clays which are about 0.80 to 2.50 m thick, and c) a few mollusc valves. Massive brownish-grey loessic material and abundant calcareous concretions appear below, between 2 and 10 m deep (Ensenada Formation). The present wetland soil develops on sediments that are 1.00 to 1.50 m thick. From the surface, it is comprised of fine sequences of dark-brown-to-black plastic and adhesive clays, abounding in iron oxide concretions. Sedimentation conditions are related to clay flocculation in an estuarine environment with distinct signs of continentalization at the end of its evolution.
3. 3 Stratigraphy
The stratigraphic units of the area are well known due to several exploratory boreholes carried out by Secretaría de Minería de la Nación and the former water supply state company Obras Sanitarias de la Nación. In particular, Plaza de Armas well reaches the crystalline basement and contains a detailed log. All depths refer to the topographic 0 IGM.
CRYSTALLINE BASEMENT (Precambrian). Composed of granites and gneiss belonging to the Brazilian Shield, the crystalline basement outcrops on Isla Martín García (50 km north of Buenos Aires) and is located in La Plata (60 km south of Buenos Aires) at -486 m deep.
OLIVOS FORMATION (Lower Miocene) (Groeber, 1945). Over 200 m thick and -486 to -271 m deep, Olivos Formation is comprised of an alternation of red sands and clays with gypsum and calcium carbonate becoming conglomeratic towards the base. Two sections can be distinguished: a) the base, composed of sand and gravel with abundant gypsum intercalations and b) the top, made up of reddish clays with carbonate and gypsum. Under arid climate conditions, the origin of this formation is continental (Aeolian-Lacustrine).
PARANÁ FORMATION (Upper Miocene) (Groeber, 1945). Up to 200 meters thick and -271 to -63 meters deep, Paraná Formation is composed of sequences of red greenish sands and marine origin clays towards the top of the formation (-154 to -63 meters). A psammitic level is found at the base (-271 to -154 meters). It is composed of medium to coarse quartz sands with marine fossils and it is 12 to 20 meters thick. The upper level is constituted by greenish bluish clays, with abundant marine fossils.
PUELCHE FORMATION (Plio-Pleistocene) (Santa Cruz, 1972). Puelche Formation is located in the well at -63 to -44 m, has unconsolidated medium to fine quartz fluvial sand, and is 15 to 25 m thick. The basal section is constituted by fine- and coarse-grained pale yellow sands. Over them there is a section of fine-grained sand with scarce magnetite.
PAMPEANO FORMATION (Upper Pleistocene) (Fidalgo et al, 1975). Up to 50 meters thick and -44 to 0 meters deep, Pampeano Formation is composed of silt and silty sand with volcanic glass and concretion of secondary calcium carbonates of aeolian and fluvial origin. These sediments outcrop in the upper parts of the area, are the components of the high plain (upper terrace) and underlie the Post-Pampeano Formation in the coastal plain.
POST-PAMPEANO FORMATION (Pleistocene-Holocene). Post-Pampeano Formation is comprised of clayey silt and sandy sediments of lacustrine fluvial and marine origin forming the coastal plain. Its occurrence is limited to fluvial valleys linked to the estuary, and its thickness grows and becomes sandier coastward.
3.4 Hydrogeology
The relationship between the geologic units and groundwater flux in the area makes it possible to individuate a three-layer aquifer system where each component of the system has well-defined characteristics.
Regarding Paraná Formation as a point of reference, the characteristics of the hydrogeologic units of the area of study can be summarised as follows.
Hydrogeologic Basement
The Crystalline Basement is the basal aquifuge unit of the aquifer systems that develop above it. It is composed of igneous and metamorphic rocks. It has been reached at different points by several organisations, for instance, at -130.8 meters a.m.s.l. in Paraná Delta and at -466.6 meters in La Plata, tilting steeply towards Río Salado basin. It acts as an impervious base of the aquifer system (Sala, 1975).
Hypo-Paraná Section
On top of Hypo-Paraná Section a continental sedimentary sequence is divided into three subsections. The best known of these subsections is the upper one, Olivos Formation, with approximately 250 meters of red sandstone and clay. This formation has various aquitard levels and some aquifers of variable salinity, which are still little known today.
The sandiest levels of Hypo-Paraná Section have aquifer potential. Although the high dissolved solids preclude its use for the most demanded application: drinking water and irrigation, it is suitable for some industrial applications. Olivos Formation carries high sulphate water with salinity contents from 6 to 60 g/l (Hernández et al, 1975).
Paraná Section
Paraná Section is located above the previous section and is of marine origin. It consists of bluish grey and green clay with sandy intercalations and abundant marine fossils. It abounds in aquiclude sediments and there are some high performance aquifer intercalations. Its thickness increases towards the south of the region and can exceed 500 meters. Moreover, discharges of up to 180 m3/h have been obtained through industrial drilling.
The central problems of exploiting Paraná Section are the aquifer depth and high water salinity, water salinity ranging from 10 to 30 g/l (Hernández et al, 1975). A pumping test performed in aquifer levels of this formation gave a transmissivity value of 5.8*10-3 m2 s-1 with a storage capacity of 1.1*10-4, values within the range of Puelche unit.
Epi-Paraná Section
Epi-Paraná Section is composed of Puelche, Pampeano and Post-Pampeano sediments.
Although its basal sections have some confining behaviour, Pampeano aquifer acts as a free aquifer. It is recharged by direct rainfall infiltration in the interfluve, being influent in terms of creeks in undisturbed areas. Besides, it constitutes the only means for receiving recharge and discharge water by vertical movement in the Puelche aquifer.
Puelche aquifer, constituted by the homonymous formation, is the main hydrogeologic unit in the area because of its surface distribution: more than 8.3*104 km2 in Buenos Aires Province (Auge y Hernández, 1983). Due to its relative shallow position, good water quality and dense population in the area, it is the most exploited aquifer in Argentina. It is used for public and industrial supply in locations that are not served by treated river water.
The water from Puelche aquifer is good quality, with less than 1 g l-1 calcium bicarbonate type, readily turning to sodium bicarbonate type by flow. Its hydraulic conductivity is about 1.5*10-5 to 5.8 m s-1 (Auge, 1990) and its storage capacity is about 0.1 to 0.05. Its regional flow moves towards Río de La Plata, with low regional hydraulic gradient.
High Plain
From a hydrogeologic perspective, the sequence above the Hydrogeologic Basement in the High Plain includes the described sections: Epi-Paraná, Paraná and Hypo-Paraná (Auge, 1995). The first one is the most superficial and encompasses Pampeano sediments and Puelche sands. These units are the most widely known and the most significant ones in hydrologic budgets.
Pampeano sediments include the phreatic table and are located most closely to the terrain surface. It is mainly composed of silt and subordinately composed of reddish brown sand and clay, frequently containing calcium carbonate concretions or banks. It is about 50 m thick. Based on previous experiments, its infiltration capacity ranges from 5 to 10 m/day and its transmissivity coefficient is approximately 200 m2/day.
Puelche sands underlie Pampeano loess and represent the most important aquifer in the northeast of Buenos Aires Province. They can be described as fine- to medium-grained quartz sands, their grains becoming bigger with depth and varying in thickness between about 20 m to 30 m in the analysed area. Average transmissivity is 500 m2/day (Auge, 1995) and water has low salinity (less than 1000 mg/l) and is therefore suitable for human consumption.
Coastal Plain
Parallel to Río de La Plata, the Coastal Plain is the last sector of a series of small streams cleaving La Plata city and its periphery. Some illustrative streams are: El Gato, Maldonado and El Pescado and their affluents (Fidalgo y Martínez, 1983). This plain is a flat environment developed between altitudes of 5 to 0 metres a.m.s.l., with mean topographic gradients of 0.5 m/km.
This monotonous, badly-drained relief, where surface watersheds do not practically occur, is locally interrupted by swells (sand levees and shell ridges) displayed parallel to the coast.
There is poor hydrogeologic information available about the Coastal Plain. Due to its high groundwater salinity, only a few perforations have gained relevant information about its hydrogeologic behaviour and environmental significance. Hence, with the aim of identifying the hydrogeologic conditions of the area of study, three boreholes were drilled. Such fieldwork will be described below.
4. MATHERIALS AND METHODS. RESULTS.
Data and information directly obtained in the area of study are described below.
Tasks performed throughout this study can be classified into two large groups:
a) Those performed only once. b) Those performed periodically.
a) The first group of tasks encompasses:
i) Geophysical survey campaigns. During these campaigns, VES and electrical imaging registers were obtained. By means of a geophysical survey it is possible to gain indirect information about the thicknesses of the formations identified in the bibliography.
ii) Construction of three deep boreholes. These boreholes reached Paraná Formation, crossing all the Puelche Formation. Contracting a private borehole drilling company was necessary for drilling them since Universidad Nacional de La Plata does not have suitable equipment for doing so.
iii) Description of cutting. It was provided during drilling operations to describe the lithological sequence.
iv) Geophysical logging in each borehole. It was performed so as to gain the required information for a correct completion of the wells.
v) Hydraulic tests. They were conducted in order to determine the hydraulic parameters of the aquifer system.
vi) A phreatimeter network. It was set up by staff from the Applied Geophysics Department of Universidad Nacional de La Plata, with the use of equipment belonging to it. These boreholes
were also logged, and a relevant hydraulic trial was carried out.
vii)Sample collection. Samples were collected for isotope determination. The isotopes determined here were 18O, 2H and Tritium.
b) The second group of tasks, those performed periodically, consists of:
i) Level measurement. It was taken monthly during the three years of fieldwork, starting in October 2006 and finishing in June 2009.
ii) Sample collection. Samples were collected for the determination of physical and chemical parameters. Over a period of three years, from October 2006 to April 2009, semestral samplings of every phreatimeter constructed in the area of study were carried out to determine main ions and some minor elements, as well as other parameters required for a correct hydrochemical interpretation.
All these tasks involved financial support, which was provided by Universidad Nacional de La Plata and the Government of the Province of Buenos Aires.
4.1. GEOPHYSICAL SURVEY
Two of the methods described in chapter 4 have been used to characterise the area of study. Vertical Electric Sounding (VES) has been applied with the aim of characterising the relevant stratigraphic section up to Paraná formation.
A technique that combines sounding and profiling, also known as 2D tomography or imaging survey, has also been used. Surface and shallow
subsurface profiles, both natural and in sites containing anthropic fill of diverse composition, have been obtained with this technique.
4.1.1 Vertical Electric Sounding
Field methodology
In the area of study, three Schlumberger VES surveys were conducted with a maximum current circuit (AB) spacing of 1,000 metres. In this type of techniques a linear tetrapolar and symmetrical device is placed at a specific observation point in the ground. The field work consists of obtaining an apparent resistivity curve by making a current I (commuted) flow through the current circuit and by measuring the potential V difference generated between the potential electrodes (MN).
On this basis, each value of apparent resistivity has been yielded (in ohm.m) as follows: I V K = ap
where K is the geometric constant of a device that takes into account the four-electrode array in the ground.
The field datum (curve of apparent resistivities) must be inverted in order to obtain a resistivity distribution with depth that mathematically satisfies the observed curve (i.e. in less than an experimental error band). Hence, as the aim of the method is to attain this resistivity distribution with depth, varied mathematical techniques are used. These techniques will be mentioned below.
Equipment
RESPC01 was the measuring equipment used here. This instrument has an automatic reading system whose results are automatically averaged. Its readings can continue until the operator is satisfied with the stability of reading. In addition, these averages are more reliable than those obtained with single reading systems.
In electric sounding mode, this instrument directly calculates ρap, showing it in Ω.m on a digital display and providing an error estimate, as well. In long sounding surveys, like the measurements taken here, the instrument measures low signals, ensures optimal penetration and has low energy consumption. It also has a microprocessor that controls and monitors measurements guaranteeing optimal sensitivity. As the microprocessor performs each measurement, it checks the circuits and the positions of switches. It verifies the condition of the battery, too. And if necessary, it can have warning systems or error codes.
Furthermore, 500m cable reels, stainless steel current electrodes and CU-SO4Cu non-polarisable electrodes were used to measure ΔV.
Processing sequence
The processing of curves was carried out with a programme for horizontal stratified media (one dimension) and following a basic sequence available in it. This software consists in the following steps: Obtaining an initial model following Zohdy (1989).
Reducing the number of model parameters (Orellana, 1982). Assessing the model equivalence and inversion with a priori
information (Pous et al, 1987).
The software works using an interactive computer system (Johansen, 1975). The model’s response in all these cases was given in a
convolutional manner using Johansen’s linear operator (op. cit.). This operator deals with a sampling of a resistivity transform of 10 points per log decade and has a length of 141 coefficients. It was considered adequate to evaluate resistive contrast curves like those occurring in the area.
Location of sounding surveys
Table 4.1.1 indicates the location of sounding surveys. Table 4.1.1 Location of VES
VES1 S 34 54 0.6 W 57 55 42.2
VES2 S 34 53 7.8 W 57 54 20.1
VES3 S 34 51 57.7 W 57 53 12.0
The table shows the geographical coordinates of plotted points, expressed in degrees, minutes and seconds (WGS 84). A plan of the location of these sounding surveys is shown below (Figure 4.1.1).
Analysis of measurements
The field device design was produced according to the expected thicknesses, considering the previous information. The measured curves are gentle curves of good quality and do not show important interferences or noise. Apparent resistivities are alternated in their morphology, making it possible to classify these curves as made up of 3/4 layers. As the series appears in different abscissa values every time, there is a variation in layer depth and thickness.
As mentioned above, the curves were first processed through an automatic fit routine following Zohdy. The result was a first model of as many layers as measuring points the curve had. The model for VES1 is shown in Figure 4.1.2.
Figure 4.1.2 VES 1
Although this is an excellent mathematical adjustment model, it lacks physical meaning so as to provide a geological designation. Therefore, a
reduced model with the lowest possible number of layers is generated in order to create a model that takes significant variations into account.
The reduced VES1 is described below (Figure 4.1.3).
Figure 4.1.3 Reduced VES 1 model.
1. The first value, up to 4 metres deep, shows surface values.
2. Between 4 and 49, there is a conductive layer corresponding to Pampeano level.
3. Between 49 and 73, there is a resistive layer ascribable to Puelche sands.
4. Finally, there is a conductive substratum attributable to Paraná formation.
The Figure 4.1.4, VES2 displays lower thicknesses and usually lower resistivity values.
Figure 4.1.4 VES 2
The Puelche layer appears at 23 metres (much higher than VES1) and resistivity reaches 10 ohm.m. That is, a higher content of salts in the formation’s interstitial water is expected.
VES3 was measured closer to the bank of Río de la Plata (Figure 4.1.5).
This curve shows an average surface value, attributable to Post Pampeano level. Then values decrease rapidly, which means the environment is much more conductive. In other words, for the same lithological formations, interstitial water has high saline contents. This interspersion between 40 and 55m is probably due to the presence of sands (equally saturated in salt water).
In short, the sections adopted are shown in Table 4.1.2.
Table 4.1.2 True resistivity sections
VES1 VES2 VES3
Depth Depth Depth 3.6 20 1.3 17.1 2.7 15 49 3.3 23.4 3.2 4.8 5.4 73 48 41.9 10.1 39.9 0.7 ∞ 8.8 ∞ 2.22 55 0.75 ∞ 0.45 RMS 5.9% RMS 3.6% RMS 3.7%
Based on these true resistivity sections, a longitudinal geoelectrical profile is shown below (Figure 4.1.6).
Figure 4.1.6 Resistivity profile
Through these sounding surveys, it is possible to identify the thickness variations of the lithological formations of a multilayer aquifer. Such soundings also indicate the evolution towards an increasingly conductive environment as the coast is reached, which can be associated with a greater content of salts in interstitial water.
4.1.2 Electric imaging survey
Field methodology
Electric profiling, in dipole-dipole array, was the technique used for constructing a subsurface imaging survey (also called tomographic image). It consists of a linear device through which resistivity variation in forward direction, at depths that can be regarded as constant, is recorded.
Apparent resistivity was calculated as follows:
ap= K
V I
where K is the geometric constant of the relevant device. Values are expressed in ohm/m.
An array with the following parameters was especially used in the survey: spacing “a” between 1m electrodes. For each station, measurements for n=1 to n=8 values were made, “n” being the different levels (with depth) of data coverage. Apparent resistivity pseudoprofiles were built up with the values thus obtained. These values provided qualitative knowledge of resistivity variations throughout the profile.
Equipment
A Scintrex IPR-12 Time Domain IP/Resistivity Receiver was used. Energization was conducted with a Scintrex IPC-9/200W source.
The IPR-12 Time Domain IP/Resistivity Receiver has an automatic reading system. The results from each reading are processed in real time and enable the operator to work on as many samples as they consider appropriate, in a number of cycles adjustable by reading. This software controls and monitors all measurements guaranteeing optimal sensitivity and good use of the instrument. While taking each measurement, the instrument checks the power supply, circuits and natural conditions.
It should be highlighted that the IPR-12 Time Domain IP/Resistivity Receiver allows simultaneous reading of up to eight potential dipoles, thus enabling interesting production times. It stores measured resistivity and chargeability values, calculates constant K values for each position and displays statistical values that indicate how reliable the stored values are.
Synchronisation between the source and the receiver is automatic and controlled by the first potential dipole. The source generates a DC signal
commuted in cycles of selectable duration. Therefore, spontaneous potential compensations can be avoided because they are statistically performed.
Processing sequence and interpretation
A two-dimensional model was made from the data already mentioned by using RES2DINV software. The 2D model used for the inversion programme consists of representing the subsurface in rectangular blocks. The block distribution fits the pseudoprofile data distribution. A forward modelling routine calculates the apparent resistivity values and an inversion routine makes use of a nonlinear least-squares optimisation technique. As the programme includes several options related to various geological features, it is possible to change its parameters in the various stages of the process. Hence, its use can be optimised in different circumstances.
The final result of the process is a model of true resistivity. An error is expressed as RMS (root mean square), a measurement of the differences between the apparent resistivity values calculated by the proposed model and those measured in the field. Once the preliminary model is found, consecutive iterations are carried out. That is, the immediate solution to the problem is achieved by minimising the RMS between the observed resistivity and the calculated apparent resistivity.
Analysis of measurements
To characterise the response of natural ground and that of anthropic fill, some sections of the area of study have been selected. The location of these sections is shown in Figure 4.1.7.
Figure 4.1.7 Profiling survey areas. Natural ground (blue) and anthropic fill (red)
The data and model obtained in natural ground is shown in Figure 4.1.8.
This response has characteristic values in 6 ohm.m, and coincided with the registered values of Pampeano level in VES surveys.
The data and model obtained from anthropic inclusions is shown in Figure 4.1.9.
Figure 4.1.9 Data and model with antrophic inclusions
where resistive inclusions can be identified.
When attempting to determine this type of inclusions, for instance because they can be regarded as contaminants, several profiles are plotted in a section.
In coincidence with the electrical profiles, geologic profiles were drawn (Figure 4.1.10). These geologic profiles were used to attribute resistivity ranges to the different materials. This area is an industrial area, where it was not possible to drill perforations. The company only authorised the use of indirect techniques Figure (4.1.11).
Figure 4.1.10 Geological Profile
Figure 4.1.11 Survey area
With the information gathered maps were drawn for each level (Figure 4.1.12).
0 50 100 150 La Pla ta Reference N i
5ohm.m 12ohm.m 18ohm.m 24ohm.m 30ohm.m 36ohm.m
Natural soil Resistive inclusions
Figure 4.1.12 Resistivity map at -2.5 meters
Interpretation contributed to characterise the distribution of various materials in a matrix equivalent to natural ground.
Two different electrical methods with different resolution capacity were used. An approximation to the problem under study was made with each of them. By means of vertical electric sounding, it was possible to
create an electrical model throughout the analysed lithological profile and also to know a priori that the interstitial water of those formations is salt-enriched towards Río de La Plata. On the other hand, with the use of imaging surveys it was possible to construct models on such a scale that resistive inclusions in an environment similar to the natural one could be identified.
4.2 GEOLOGICAL PROFILE
In order to verify the geological conditions in situ, 3 boreholes reaching Paraná Formation were drilled. They were then cased to be used in physical and chemical monitoring of water.
The vicinity of El Dique (PU01), Tiro Federal (PU02) and prefecture premises in Puerto La Plata (PU3) were defined as appropriate sites for drilling these boreholes.
Drilling tasks began with a stratigraphic reconnaissance survey. In a two-inch diameter, cutting (lithological) sampling was done (metre to metre) until the base of Puelche aquifer was determined.
According to the results drawn from the borehole, two definitive boreholes, corresponding to each of the monitored aquifers, were designed in each site.
The borehole was widened to a six-inch diameter, up to the least permeable layer located above the roof of Puelche aquifer. Casing was run in to this level with PVC pipes of 110 mm in diameter. Cementation in the annular space and bottom of the borehole was, in turn, carried out.
Cement was set adequately, in 24 hours. After the cement plug was drilled out until it reached the final depth defined in the design, drilling continued with a four-inch diameter. Such depth was the upper section of the aquifer in all three cases.
Subsequently, a stainless steel Johnson’s screen with an opening of 0.25 mm was placed with its extension pipe and pipe organisers, as well as a gravel annular fill. Gravel (2–4 mm) was introduced in the annular space between the formation and the external side of pipes.
Variable-rate pumping was used for borehole development and cleaning. Once the water withdrawn showed crystalline features, this process was over.
In order to construct the definitive monitoring wells in the Pampeano aquifer, drilling took place between a six-inch diameter and the established final depth.
PVC casing was subsequently run. The PVC pipelines were 110 mm in diameter and slotted in their lowest section. Gravel was inserted up to a top layer of low permeability and a rubber pack was then placed with the aim of isolating this material from the phreatic layer.
Finally, the borehole was cleaned through variable-rate pumping.
The Figures 4.2.2, 4.2.3 and 4.2.4 show a basic design of the boreholes and the lithology crossed by each.
Figure 4.2.3 Description of borehole PU02
Figure 4.2.5 shows a well log measured in PU01 borehole. The record values are typical for the formations:
Figure 4.2.5 El Dique well log record
For Pampeano sediments around 50 cps in natural gamma record, and 10 ohm.m for resistivity tools; for clay 80 cps in gamma record and 6 ohm.m for resistivity tools; 20 cps in gamma record and 50 ohm.m in resistivity tools for Puelche sands (Ainchil et al, 2007)
Figure 4.2.6 shows a hydrogeological profile that links the boreholes mentioned above, displaying the position of the hydrogeological units and their thickness variations in the area.
Figure 4.2.6 Geological profile
Puelche sands, which represent the most important aquifer in the northeast of Buenos Aires Province, include a sequence of usually well-selected yellowish brown quartz sands. They overlie bluish green clays of Paraná Formation. This aquifer roof consists of fine-grained silty sands (grain size increasing with depth) that become coarser towards the base. Thickness slightly decreases from the continent to the coast because it is 14 meters in El Dique, 13 meters in Tiro Federal and 10 meters in Puerto La Plata.
The contact between Pampeano sediments and Puelche sands is clear in El Dique and Tiro Federal boreholes: in the former, through about 5 meters thick grey clay, and in the latter, through about 2 meters grey clay. However, in Puerto La Plata, once the typical sand is defined, there is no more gradual variation in grain size.
Pampeano sediments are the base of Post-Pampeano ones. The latter are composed of grey or greenish clayey and sandy silts of marine estuarine origin. They usually form a low-permeability unit, with some fine-grained sandy silt intercalations of higher permeability values that
